1. Field of the Invention
The present invention relates to a scanner having a background assembly, and more particularly to a scanner having a light-obstruction member for blocking a portion of light beams from reaching a reference backing member which is used for calibration of scanner characteristics and for foreground detection.
2. Related Art
In a conventional scanner, a white calibration sheet is usually provided to facilitate the calibration, detection and compensation of the characteristics of a light source and an image sensor of the scanner. However, the white calibration sheet cannot facilitate detecting the foreground of a document with a white edge, and thus the scanned image quality is affected. In a sheet-fed scanner, a scanning module is stationary, and the white calibration sheet is disposed above a scan window and at a scan position. If the light source emits light beams from an upstream side of the scan position, then the image representative of the leading edge of the document is shadowed when the leading edge almost reaches the scan position. Likewise, if the light source emits the light beams from a downstream side of the scan position, then the image representative of the trailing edge of the document is shadowed when the trailing edge just leaves the scan position. Because the document blocks a portion of the illumination of the light source on the white calibration sheet and the light beams reflected by the white calibration sheet and received by the image sensor is lower than a threshold value, when the scanned image is being processed, the image of the white calibration sheet is thus determined as one portion of the image of the leading edge of the document. As a result, a dark shade would be produced at the front section or rear section of the scanned image of the document, and the scanned image could not faithfully represent the original image of the document.
Most documents have white edges, and a black background sheet is provided to facilitate detecting the edges of the document and obtaining reference for document size detection and image cropping. If the functions of characteristics calibration and edge detection are to be integrated, a switchable background sheet is usually employed in the prior art. The background sheet can be switched to the white background when the scanner is being calibrated, and the background sheet can be switched to the black background for edge detection.
Although the switchable background sheet can accomplish both functions of the edge detection and scanner characteristics calibration, more components are required in order to switch the background sheet. Thus, the size of the scanner is increased and the design of the conventional scanner is relatively complicated.